Conventional rotary-wing aircrafts use a set of complex mechanical gearboxes that convert the high rotation speed of gas turbines into the low speed/high torque required to drive the rotor(s). Unlike powerplant, mechanical gearboxes cannot be duplicated for redundancy, hence remain a known single point of failure (SPOF) consistently affecting helicopter reliability and safety. In-flight catastrophic gear failure usually results in gearbox jamming and subsequent fatalities, whereas loss of lubrication can trigger onboard fire. This fact is eloquently documented by regular crash reports issued by government bodies in charge of civil and military aviation (e.g. EASA, BASI and NTSB). Furthermore, a well-known operational weakness of mechanical gearboxes is their inherent transient power limitation due to material and structural fatigue limits. Gears are the most vulnerable to fatigue. Aerial work operations such as sling load, long line and helicopter logging experience the highest rate of crashes and fatalities resulting from fatigue induced transmission failure.
Another limitation of mechanical gearboxes is their fixed reduction ratio, precluding fuel saving during cruise when the main rotor speed of helicopters could be slightly reduced in view of lowering aerodynamic losses.
Electric drive trains have recently been integrated in light recreational airplanes in the form of small powerplant. However, those propulsion systems are usually developed around standard off-shelf components. The usual split configuration includes one or more electric motor(s) powered by one or more separate controller(s) altogether managed by an external control unit. Beyond the fact that such architecture is littered with single points of failure (SPOF) leading to unacceptable failure rates as per commercial aviation standards (1.10-4 to 1.10-5 failure/hour, for an objective better than 1.10-7), the main problem resides in the poor electromagnetic compatibility with the surrounding environment (commonly failing MIL-STD-461/462 and EMP standards). Fast switching power circuitry (such as IGBT and/or MOSFETS) commonly used in motor controller to minimize power losses is one cause of the electromagnetic compatibility problem. Such circuitry produces high order harmonics, hence significant interference with surrounding avionics and onboard electronic systems. Interferences are generated by the controller units in two forms: Radiated Emissions (RE) and/or Conducted Emissions (CE). The former refers to free space propagation of electromagnetic radio waves, whereas the later refers to electromagnetic signals propagating along the power lines and data cables, potentially disturbing the operation of aircraft's systems. Radiated emissions suppression usually requires installing heavy metallic shielding around the controller unit, whereas conducted emissions are mitigated by using inherently heavy inline filters inserted in the controllers' DC power ports, combined with shielded cables. Additionally, a multitude of inline filters needs to be installed on the inputs/output ports (power supply and data lines) of each avionic system operating in the vicinity of the electric powerplant. If those somewhat heavy fixes can mitigate the detrimental effect of conducted emissions, they have a very negative impact on system's weight.
In addition to generating interferences, such architecture is vulnerable to onboard and/or external electromagnetic interferences. Onboard interferences are generated by avionics systems such as radar transponder, DME, radio-altimeter, weather radar, HF/VHF/UHF transceivers, ECM/ECCM (electronic counter measures and electronic counter-counter measures), whereas external interferences may originate from a wide variety of sources such as EMP weaponry, high altitude nuclear detonation, ECM attack, strong RF signals from ground RADAR, electromagnetic beam weaponry or other sources. There have been reported cases of in-flight motor stoppage resulting from onboard avionics interference, or when flying in the vicinity of radio transmission towers.
The foregoing problems present a major limitation to the operational use of electric propulsion technologies in aircraft. From a military standpoint, current electric propulsion systems are unacceptably vulnerable and cannot be deployed on the modern battlefield.
Lastly, the vast majority of today's electric drives do not meet hardening requirements against lighting strike and electrostatic discharges (ESD). In their present form, electrical drives are inherently vulnerable and unable to survive such event, whereas aircrafts operating in IFR (Instrument Flight Rules) condition must withstand direct lightning strike and strong ESD discharges.
In any avionics system, data lines are the most vulnerable to external RF interference and ESD discharges as they carry small signals and connect to the most sensitive and fragile components. The present invention discloses a fault tolerant direct drive system, inherently more reliable than mechanical gearboxes, offering better mechanical fatigue resistance, as well as being robust against electromagnetic interferences, EMP and ESD aggressions.